Pulse control circuit for a dc load

ABSTRACT

A circuit for providing controlled pulses of direct current to vary the speed of a direct current motor. A main controlled rectifier applies current to the motor. A commutation circuit consisting of an inductor, a capacitor and another controlled rectifier turns off the main controlled rectifier by reversing its polarity. Another circuit, referred to as a &#39;&#39;&#39;&#39;ring-around&#39;&#39;&#39;&#39; circuit, consisting of another inductor and controlled rectifier reverses the polarity of the commutating capacitor preparatory to the commutation of the main controlled rectifier.

United States Patent 1 1 1 1 3,784,890 Geiersbach et al. Jan. 8, 1974 [54] PULSE CONTROL CIRCUIT FOR A DC 3,582,764 6/l97l Huber 321 45 c x LOAD 3,6l9,753 11/1971 Thompson 321/45 C X [75] Inventors: grgg f fifi z zgimi tzii of Primary ExaminerBemard A. Gilheany Wis Assistant Examiner-Thomas Langer Attorney.lohn P. Hines et al. [73] Assignee: Allis-Chalmers Corporation, 1

Milwaukee, WIS. ABSTRACT 2 F1 (1: 2 1972 [2 1 l 6 Dec 1 A circuit for providing controlled pulses of direct curl l pp ,341 rent to vary the speed of a direct current motor. A main controlled rectifier applies current to the motor. [52] U s C] 318/345 3321/45 c A commutation circuit consisting of an inductor, a ca- [51] i102) 5/16 pacitor and another controlled rectifier turns off the [58] Field '7 694 341 main controlled rectifier by reversing its polarity. An-

. I 318/345: 321745 other circuit, referred to as a ring-around circuit, consisting of another inductor and controlled rectifier [56] References Cited reverses the polarity of the commutating capacitor preparatory to the commutation of the main con- UNITED STATES PATENTS trolled rectifier. 3,518,520 6/!970 Molnar 318/341 X 3,562,611 2/1971- Gurwicz 318/341 X 4 Claims, 1 Drawing Figure TO 01 1: CIRCUIT SCFEB SC TO ON CIRCUIT TO POLARITY REVEQSAL CIRCUIT LI SCR2 IIIIIII *SCRI TO ON CIRCUIT TO OFF CIRCUIT SUB 1 PULSE CONTROL CIRCUIT FOR A DC LOAD This invention pertains in general to improvements relating to control means for electrical apparatus and more particularly to control means for providing controlled pulses of direct currentto a direct current load.

The general purpose and object of the control of this invention is to provide smooth, stepless voltage control to a DC series motor which in turn provides smooth control of the motor speed. A method of control known as pulse width modulation (PWM) is utilized. A solid state switch, in this instance a silicon controlled rectifier (SCR) 'is used to switch the supply voltage on and off the motor rapidly. The ratio of on-time to off-time determines the average motor-voltage and therefore the motor speed. The control utilizes a constant switching rate which is akin to frequency and varies the ontime to off-time ratio to achieve voltage control.

One particular circuit embodying the improvements according to the present invention which performs this time ratio PWM control is shown in FIG. 1.

A description and identification of each component in the preferred circuit shown in FIG. 1 is given below:

SCRl This is the main SCR which carries the motor current and is switched on and off to perform the PWM control. 1

SCR2 This SCR is used in conjunction with inductor L1 to reverse the polarity of the commutating ca-' pacitor C. It is referred to as the ring-around SCR.

SCR3 This is the commutating SCR which applies voltage on the commutating capacitor C across the main SCRI to turn it off.

C This is the commutating'capacitor which stores energy to commutate the main SCRl.

Ll This is the ring-around inductor used in conjunction with SCR2 to reverse the polarity of capacitor C.

L2 This inductor limits the rate of rise of current in SCR3 and also along the inherent inductance in the battery overcharges the commutating capacitor C.

D This is referred to as a free wheeling diode which carries motor current during the off-time of SCRl.

An SCR- is considered forward biased when its anode is positive with respect to its cathode. It will conduct current in only one direction when a gate current is supplied if it has previously been forward biased. An SCR is turned off when its anode is negative relative to its cathode and when in this condition the SCRis said to be reverse biased. Once the SCR is reverse biased or commutated, it must be forward biased before it can be turned again by supplying a gate current.

Referring to the circuit diagram, the operation of the control will now be described. .The circuit switch is closed and a gate current signal from the off circuit is applied to SCR3 which is forward biased causing SCR3 to turn on. The current then flows through the motor, the capacitor C, inductance L2, and the SCR3 causing the capacitor to charge to battery voltage with a forwardpolarity as shown in the drawing. Only a very short period of time is required to charge the capacitor. Once the capacitor is charged to the battery voltage, currentflow stops.

At this point the main SCRl is gated on, causing battery current to flow through the motor and the SCR]. As soon as SCRl is gated on, the voltage across capacitor C is impressed across SCR3, reverse biasing SCR3,

causing SCR3 to turn off. A short time is allowed to elapse after the gating of SCR] to assure proper commutation (turn-off) of SCR3 and then SCR2, which is forward biased, is gated on.

By turning SCR2 on, the voltage across the capacitor C causes current to flow through SCR2 and inductor Ll. That is, the energy stored in capacitor C is transferred to inductor L1 and then is transferred back to capacitor C. In the process of this energy transfer the polarity across capacitor C is reversed. This condition or process of capacitor reversal is referred to as ringaround." After ring-around is complete, that is the capacitor C is fully charged in the reverse polarity, current ceases to flow in the ring-around circuit. SCR2 is automatically commutated off because of the reverse bias of its polarity after ring-around is completed. Furthermore, due to the reverse bias of capacitor C relative to that shown in the circuit diagram, the SCR3 is forward biased back to its original condition where it is again ready to be gated on.

Once the capacitor C is reversed in polarity and SCR2 is commutated off SCRl can be turned off. The total on-time of SCRl is, of course, controlled by the operator depending on whether he wants slow speed (short on-time) or high speed (long on-time). In either case, turn off of SCRl is accomplished by gating on SCR3 which applies the commutating capacitor C across SCRl reversing its polarity and effectively commutating SCRl. Load currentthrough the motor continues to flow through the capacitor, inductance L2 and SCR3 until the capacitor is charged back to its original polarity as shown in the circuit diagram. Load current then continues to flow through free wheeling diode D.

The capacitor is actually able to charge to a voltage higher than the battery voltage because the energy stored in the battery and line inductance and inductor L2 is transferred to energy in the capacitor causing the extra voltage to be stored therein. At higher load currents this over voltage charge on the capacitor is significant, causing a final voltage on the capacitor significantly greater than battery voltage. This is helpful since it provides extra commutation energy needed to turn SCR] off at higher currents.

SCR3 is commutated off in one of two ways. First, at higher load currents, capacitor C is able to charge to a voltage higher than battery voltage. When the energy in the line and battery inductance and inductor L2 has been totally transferred to capacitor C, the capacitor will try to force current backthrough diode D, the battery, SCR3 and L2 because it is at a higher voltage than the battery. As soon as this happens, SCR3 becomesreverse biased by a voltage equal to the difference between the battery and capacitor voltage causing it to turn off. Secondly, under light load currents and low percent on times for SCRI, current may still be flowing in capacitor C when SCR] is turned back on and C may not yet be fully charged back to battery voltage. Under these conditions, when SCR] is turned on again it automatically applies whatever voltage is across capacitor C across SCR3, reverse biasing it and causing it to turn off.

During the off period of SCR] the motor current flows through the free wheeling diode D until SCRl is again gated on. After the desired period of off-time SCRl is again gated on andthe cycle repeats itself.

The embodiments of the inventionin which an exclusive property or privilege is claimed are defined as follows:

l. A control system for a direct current operated load comprising;

a first controlled rectifier connected in series between a terminal of the load and the direct current source;

second and third controlled rectifiers and first and second inductors all connected in series with said load and across said firstcontrolled rectifier;

a capacitor connected in parallel with said second controlled rectifier and said first inductor and in series with said third controlled rectifier, said second inductor, and said load;

ers. 

1. A control system for a direct current operated load comprising; a first controlled rectifier connected in series between a terminal of the load and the direct current source; second and third controlled rectifiers and first and second inductors all connected in series with said load and across said first controlled rectifier; a capacitor connected in parallel with said second controlled rectifier and said first inductor and in series with said third controlled rectifier, said second inductor, and said load; and gating signal sources connected to each of said controlled rectifiers.
 2. The control system set forth in claim 1 wherein said direct current load is an electric motor and further comprising a rectifier diode connected across the motor to permit motor current to flow during the off period of said first controlled rectifier.
 3. The control system set forth in claim 1 wherein said controlled rectifiers are silicon controlled rectifiers.
 4. The control system set forth in claim 2 wherein said controlled rectifiers are silicon controlled rectifiers. 